The reaction by which formaldehyde is obtained from methanol by catalytic oxidation has been known since 1878 and the use of silver catalysts for this purpose has been known at least since 1908 as is disclosed in German Patent No. 228,687. Though other catalytic metals and metal oxides have been proposed and used, the use of silver for this process is quite widespread.
There are two commercially accepted processes. The first utilizes a silver catalyst and operates in an oxygen deficient atmosphere. The second utilizes a metal oxide catalyst and operates in a methanol deficient atmosphere. The first method for carrying out the process involves passing a mixture of methanol vapor and air over a stationary catalyst at approximately atmospheric pressure and absorbing the product gases in water. The mechanism is believed to be a combination of two reactions involving the dehydrogenation and oxidation of methanol: EQU CH.sub.3 OH.fwdarw.HCHO+H.sub.2 EQU CH.sub.3 OH+1/2 O.sub.2 .fwdarw.HCHO+H.sub.2 O.
It is desirable to provide a catalyst which is very selective in promoting the formation of formaldehyde while at the same time minimizing side reactions which result in the formation of CO and CO.sub.2.
Silver-catalyzed processes for making formaldehyde from methanol can be characterized according to the number of catalytic stages used to effect the conversion. Single stage operation is quite widely used but suffers from the disadvantage that rather high amounts of unconverted methanol are contained in the product emerging from the catalyst bed. This phenomenon is customarily referred to as "methanol leakage". Since for many applications methanol is an undesirable contaminant, it must be separated from the formaldehyde solution. This entails a substantial investment in distillation facilities and energy to carry out such separations. It is usually necessary that the methanol content of the product be no greater than 2% by weight.
One way of eliminating the need for facilities to distill off methanol is to use two catalytic stages with interstage cooling. A basic two-stage process of this type was disclosed in U.S. Pat. No. 2,462,413 to Meath. In Northeimer's U.S. Pat. No. 3,959,383, an improvement on the Meath process is disclosed by which even lower amounts of methanol in the product can be obtained.